Electrophilicity charge

Pyrrole is very reactive towards electrophiles charge distribution from the nitrogen makes either C-2 (or C-3) electron rich. Thus, a second porphobilinogen acts as the nucleophile towards the methylidene pyrrolium cation in a conjugate addition reaction. It is now possible to see that two further identical steps will give us the required linear tetrapyrrole, and that one more time will then achieve ring formation. 

It is the charge of an orbital. Electrophilic charge q and nucleophilic charge q are... 

Acyl cations are relatively weak electrophiles. This is easily understood, because their structure is of a predominantly linear carboxon-ium ion nature, with the neighboring oxygen atom delocalizing charge and limiting their carbocationic nature. 

Charge-charge repulsion effects in protolytically activating charged electrophiles certainly play a significant role, which must be overcome. Despite these effeets multidentate protolytic interactions with superacids can take plaee, increasing the electrophilie nature of varied reagents. 

As well as the cr-complexes discussed above, aromatic molecules combine with such compounds as quinones, polynitro-aromatics and tetra-cyanoethylene to give more loosely bound structures called charge-transfer complexes. Closely related to these, but usually known as Tt-complexes, are the associations formed by aromatic compounds and halogens, hydrogen halides, silver ions and other electrophiles. 

The model adopted by Ri and Eyring is not now acceptable, but some of the more recent treatments of electrostatic effects are quite close to their method in principle. In dealing with polar substituents some authors have concentrated on the interaction of the substituent with the electrophile whilst others have considered the interaction of the substituent with the charge on the ring in the transition state. An example of the latter method was mentioned above ( 7.2.1), and both will be encountered later ( 9.1.2). They are really attempts to explain the nature of the inductive effect, and an important question which they raise is that of the relative importance of localisation and electrostatic phenomena in determining orientation and state of activation in electrophilic substitutions. 

The electrostatic interaction of the charge on the orienting substituent, and those at the nuclear positions, with that of the approaching electrophile. 

Consideration of (i), as in the work of Ridd and his co-workers, would constitute a transition state theory of the substituent effects. (2) alone would give an isolated molecule description, and (3), in so far as the charge on the electrophile was considered to modify those on the... 

Hiiekel m.o. ealenlations fail badly with benzimidazole. Loealisation energies for the free base and the eation indicated C(4) to be the most reactive position towards electrophilic attack, - and led to the false conclusion that substitution involved the free base, the orientation being controlled by charge densities. 

Ambident reactivity occurs for any substance displaying protomeric behavior. In basic medium anions in which negative charge is delocalized are formed this may be represented by the resonance formulas in Scheme 2. Each charged atom may react with an electrophilic center. The... 

The charge diagram of A-4-thiazoline-2-one is summarized in Table VII-9. This diagram and the one obtained by a HMO treatment (105) are consistent with the easy acetylation occurring in position 5 of the ring. However, PPP calculations indicate that this electrophilic substitution could also have occurred in position 4, which is not observed. 

Reaction takes place on nitrogen when the electrophilic center is an sp carbon, particularly if it is charged. Thus Mannich reaction yields the N-substituted compound (71 and 72) (Scheme 34) (54. 157-159). The same reaction is reported with piperidine, o-toluidine. and methylaniline (158). 

The lack of examples demonstrating the reactivity on C-2 may be the misleading impression that this atom is not electrophilic, contrary to what is indicated from charge diagrams. Such is not the case as the Cook s rearrangement demonstrates (209, 212). A logical mechanism for this reaction involves the tetrahedral intermediate (88) (Scheme 42). This... 

The positive charge on carbon and the vacant p orbital combine to make carbo cations strongly electrophilic ( electron loving or electron seeking ) Electrophiles are Lewis acids (Section 117) They are electron pair acceptors and react with Lewis bases (electron pair donors) Step 3 which follows and completes the mechanism is a Lewis... 

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

The electrophile (E ) m this reaction is mtromum ion (0=N=0) The charge distn bution m mtromum ion is evident both m its Lewis structure and m the electrostatic potential map of Figure 12 2 There we see the complementary relationship between the electron poor region near nitrogen of NO, and the electron rich region associated with the TT electrons of benzene... 

The electrophilic site of an acyl cation is its acyl carbon An electrostatic poten tial map of the acyl cation from propanoyl chloride (Figure 12 8) illustrates nicely the concentration of positive charge at the acyl carbon as shown by the blue color The mechanism of the reaction between this cation and benzene is analogous to that of other electrophilic reagents (Figure 12 9)... 

Turning now to electrophilic aromatic substitution in (trifluoromethyl)benzene we con sider the electronic properties of a trifluoromethyl group Because of their high elec tronegativity the three fluorine atoms polarize the electron distribution m their ct bonds to carbon so that carbon bears a partial positive charge... 

Oxygen stabilized carbocations of this type are far more stable than tertiary carbocations They are best represented by structures m which the positive charge is on oxygen because all the atoms have octets of electrons m such a structure Their stability permits them to be formed rapidly resulting m rates of electrophilic aromatic substitution that are much faster than that of benzene... 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

C 1 IS more reactive because the intermediate formed by electrophilic attack there IS a relatively stable carbocation A benzene type pattern of bonds is retained m one nng and the positive charge is delocalized by allylic resonance... 

The regioselectivity of substitution in furan is explained using a resonance descrip tion When the electrophile attacks C 2 the positive charge is shared by three atoms C 3 C 5 and O... 

When the electrophile attacks at C 3 the positive charge is shared by only two atoms C 2 and O and the carbocation intermediate is less stable and formed more slowly... 

The active form of the catalyst having one less ligand and being positively charged acts as an electrophile toward ethylene m Step 2... 

Electron distribution governs the electrostatic potential of molecules. The electrostatic potential describes the interaction of energy of the molecular system with a positive point charge. Electrostatic potential is useful for finding sites of reaction in a molecule positively charged species tend to attack where the electrostatic potential is strongly negative (electrophilic attack). 

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